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Print ISSN: 0355-3140 Electronic ISSN: 1795-990X Copyright (c) Scandinavian Journal of Work, Environment & Health

Downloaded from www.sjweh.fi on February 14, 2012

Original article

Scand J Work Environ Health 2012;38(1):70-77

doi:10.5271/sjweh.3194

Estimation of life expectancies and loss-of-life expectancies

for workers with permanent occupational disabilities of the

extremities a 21-year follow-up study

by Lin S-H, Lee H-Y, Chang Y-Y,Jang Y, Wang J-D

Affiliation: Department of Public Health, College of Medicine,

National Cheng Kung University, No. 1, University Road, Department

of Public Health, College of Medicine, National Cheng Kung

University, Tainan 70101, Taiwan. [email protected]

Refers to the following texts of the Journal: 1982;8(3):153-158

2006;32(2):91-99 2002;28(5):328-332

Key terms: amputee; disability; life expectancy; loss-of-life

expectancy; lower extremity; occupation; occupational disability;

permanent disability; prevention; worker

This article in PubMed: www.ncbi.nlm.nih.gov/pubmed/21912828
http://d/www/www.sjweh.fi/show_issue.php?issue_id=291http://d/www/www.sjweh.fi/show_abstract.php?author_id=6540http://d/www/www.sjweh.fi/show_abstract.php?author_id=6541http://d/www/www.sjweh.fi/show_abstract.php?author_id=6542http://d/www/www.sjweh.fi/show_abstract.php?author_id=6543http://d/www/www.sjweh.fi/show_abstract.php?author_id=233http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=2480http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=984http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=682http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7249http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=778http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=3343http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=2243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=74http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7244http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7245http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=378http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=325http://www.ncbi.nlm.nih.gov/pubmed/21912828http://www.ncbi.nlm.nih.gov/pubmed/21912828http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=325http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=378http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7245http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7244http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=74http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=2243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7243http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=3343http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=778http://d/www/www.sjweh.fi/show_abstract.php?keyword_id=7249http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=682http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=984http://d/www/www.sjweh.fi/show_abstract.php?abstract_id=2480http://d/www/www.sjweh.fi/show_abstract.php?author_id=233http://d/www/www.sjweh.fi/show_abstract.php?author_id=6543http://d/www/www.sjweh.fi/show_abstract.php?author_id=6542http://d/www/www.sjweh.fi/show_abstract.php?author_id=6541http://d/www/www.sjweh.fi/show_abstract.php?author_id=6540http://d/www/www.sjweh.fi/show_issue.php?issue_id=291

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70 Scand J Work Environ Health 2012, vol 38, no 1

Original articleScand J Work Environ Health 2012;38(1):7077. doi:10.5271/sjweh.3194

Estimation o lie expectancies and loss-o-lie expectancies or workers withpermanent occupational disabilities o the extremities a 21-year ollow-up

studyby Sheng-Hsuan Lin, MD,1 Hsin-Yi Lee, PhD,2Yu-Yin Chang, MSc,2Yuh Jang, PhD,3Jung-Der Wang,MD, ScD2, 4, 5

Lin S-H, Lee H-Y, Chang Y-Y, Jang Y, Wang J-D. Estimation of life expectancies and loss-of-life expectancies forworkers with permanent occupational disabilities of the extremities a 21-year follow-up study. Scand J WorkEnviron Health. 2012;38(1):7077. doi:10.5271/sjweh.3194

Objectives This study aims to estimate the life expectancies and loss-of-life expectancies of workers withpermanent occupational disabilities of the upper and lower limbs in Taiwan.

Methods We collected all cases of permanent occupational disability in the upper and lower limbs from theBureau of Labor Insurance database of compensation claims between 19862006; these data were linked with

the national mortality registry to obtain a survival function. Workers were divided into eight groups according

to their injury types, three of which fulfilled the condition of constant excess hazard and the survival functions

were extrapolated to 50 years using a semi-parametric method.

Results Of the subjects involved in the study, 1016 with toe amputations, 995 with foot or leg amputations, and4339 with foot or leg non-amputations showed a life expectancy of 1.8 [95% confidence interval (95% CI) -1.3

-4.9], 4.9 (95% CI 2.2 -7.6), and 4.5 (95% CI 2.1 -6.9) years, respectively. The above method was validated

by extrapolating partial cohorts based on the first 10 years of follow-up data to 21-year and comparing actual

survival rates using the Kaplan Meier method. The relative bias of three groups was

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Scand J Work Environ Health 2012, vol 38, no 1 71

Lin et al

HIV (5) or cancer (6, 7). That method was also applied

in the estimation of life expectancy for workers with

permanent occupational disabilities from 19862000

and demonstrated that the expected loss-of-life expec-

tancy varied between 519 years, depending on the

grade of major physiological dysfunction and injury

type (8, 9). However, the above results cannot be

directly applied to workers with less severe, more

specific impairments such as amputation of the upper

and lower extremities. The aforementioned cohort was

followed up through 2006, providing an opportunity

to evaluate the long-term survival of these physically

challenged workers. The objective of this study was to

estimate the life expectancy and loss-of-life expectancy

of workers with permanent occupational disabilities of

the lower and upper extremities, including amputees

and non-amputees.

Methods

Study population

In Taiwan, the Bureau of Labor Insurance (BLI) has

administered a compensation scheme for workers

since 1950. A historical prospective study was carried

out based on the computerized registry established by

the BLI, which included all cases of permanent dis-

ability from 19862006 that resulted in compensation.

According to statements released by the BLI in Taiwan,the term approved compensation claim for permanent

occupational disability (ACCPOD) indicated that the

disabilities of affected workers were caused by occupa-

tional injuries or diseases and that they were unable to

recover within one year under medical care, regardless

of whether they were able to return to work (www.bli.

gov.tw/File.aspx?uid=NS%2B%2Fve99M9Q%3D ).

In total, the computerized registry consisted of 106

437 compensation claims for permanent occupa-

tional disability resulting from work-related injuries

and included details of workers gender, age on date

of injury, type of disability and accident, body partaffected, and monthly insured salary. Initially, 4745

cases were excluded due to incomplete information

on gender, age on date of injury, or body part affected.

Because the primary aim of this study was to determine

whether people with single injuries of the extremities

have reduced life expectancies, we only included sub-

jects with a single disability in our analysis. Thus, we

excluded 18 605 cases of individuals with more than

one type of disability. Among these excluded cases,

12 237 were compensated in one single application,

while the other 3049 subjects suffered from 6368 times

of recurrent injuries. We also excluded 11 907 cases

that involved injury locations other than the upper or

lower extremities. As summarized in figure 1, a total

of 71 001 cases remained for our analysis.

Among those individuals included in this study,

8017 suffered from injuries of the lower extremities and

62 984 suffered from injuries of the upper extremities.

We further stratified these cases into four categories of

injury: toes only, leg or foot, fingers, and hand or

arm. Each of these four categories was further divided

into amputee and non-amputee groups, which resulted in

a total of eight groups analyzed in this study. According

to the BLI, an amputation is defined as more than half

of a segment cut off; patients with all other kinds of dis-

ability that did not result in amputation were classified

as non-amputatees. The survival status of the individual

in each case was tracked through the end of December

2006 and was verified with the National Mortality

Registry (NMR) in Taiwan. The NMR, established in

1950, is maintained by the Department of Health of

the Executive Yuan (or institute). It is the most reliable

official database for certificates of death in Taiwan and

has been computerized since 1981 (10). In this study, we

linked the personal identification (ID) number of com-

pensated individuals from the BLI registry to the NMR

database. If a worker was found to have a record in the

mortality registry under the same ID number, then the

date and cause of death could be accurately confirmed.

If this verification was not possible, the individual was

considered to be either alive or to have had his or her

record censored sometime prior to 31 December 2006.

Because the annual emigration rate for Taiwan wasgenerally below 0.1% from 19862000, and most emi-

grants still pay their premiums and keep their insurance

under the National Health Insuranceprogram due to its

comprehensive coverage, the loss to follow-up among

censored subjects was very low after comprehensive

linkage with the data of NMR. Foreign workers were

not included in this study because they have not been

given ID numbers and hence their survival status could

not be verified.

Method o extrapolating survival unctions to lietime

After the aforementioned procedure, we obtained the

survival functions of eight different groups. As most

of these workers survived past the 21-year follow-up

period, we adopted a method of extrapolation to esti-

mate the mean and 95% confidence interval (95% CI)

of the lifetime survival for different types of disability.

The general method of extrapolation used in this study

was to assume that a disease can produce premature

mortality (or excess hazard), which can be quanti-

fied based on the follow-up of the patient cohort for a

shorter period of time (4, 11). If the excess mortality

in the study population appears to be a constant hazard
http://www.bli.gov.tw/File.aspx?uid=NS%2B%2Fve99M9Q%3Dhttp://www.bli.gov.tw/File.aspx?uid=NS%2B%2Fve99M9Q%3Dhttp://www.bli.gov.tw/File.aspx?uid=NS%2B%2Fve99M9Q%3Dhttp://www.bli.gov.tw/File.aspx?uid=NS%2B%2Fve99M9Q%3D

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Loss-of-life expectancy for occupational extremity disability

compared with the age- and gender-matched reference

population during the first several years of follow-up,

we then use information from the reference population

for further extrapolation. We applied the Monte Carlo

method to generate the lifetime survival function for this

matched reference population by incorporating data (or

hazard functions) from national vital statistics. Finally,if the assumption of constant excess hazard holds,

then the logit-transformed survival ratio, indicated

by W(t), between the study and reference population

becomes linear during the later follow-up period, and

the estimated regression line can be used to extrapolate

lifelong survival beyond the follow-up period (5). We

have simulated (11) and mathematically proven(5) that

this is a valid method for predicting the life expectancy

under a high censored rate,which is also corroborated

by several real examples (5, 7, 9).

In order to verify whether the above method of

extrapolation can be appropriately applied to workerswith permanent occupational disabilities of the upper

and lower extremities, we first checked the temporal

trend of logit W(t) during the first 21 years in each

group. For each subject with a permanent disability,

100 age- and gender-matched referents were simulated

from the life tables of vital statistics of Taiwan using

the Monte Carlo method. If the hazard function for the

population with permanent disabilities of the upper and

lower extremities is proportional to that of the reference

population at each time t, then the logit transformation

of the ratio W(t) will become linear after the follow-up

period; the slope of that line can then be used to extrapo-

late long-term survival beyond the follow-up period.

This sampling and estimation procedure was repeated

100 times to obtain 100 projected estimates of survival

and 100 bootstrap standard errors.

Validation o the Monte Carlo extrapolation

The validation of the actual performance of the Monte

Carlo extrapolation for workers with permanent occu-

pational disabilities of the upper and lower extremities

was performed on a sub-cohort of the study popula-

tion that included workers with occupational injuries

occurring between 19861995. Assuming that those

patients were only followed until the end of 1995 (for

the first ten years), we extrapolated their survival data

through the end of 2006 using the Monte Carlo method.

Because this sub-cohort was actually followed until

the end of 2006, the actual observations at the end

of the 21-year follow-up period calculated with the

Kaplan-Meier method were considered the gold stan-

dard. In addition, we defined relative bias, which is

the indicator of the accuracy of our estimation, as the

value of difference between the actual observations

calculated using the Kaplan-Meier method and the

Monte Carlo simulation results, divided by the Kaplan-

Meier estimate.

We also defined health gap as the value of loss-of-

life expectancy of the cohort of interest divided by the

life expectancy of the age- and gender-matched general

population.

Results

Characteristics o subjects

As summarized in table 1, the studied population groups

were male-dominant; the proportions of male worker

ranged from 74.087.6%. The mean age for each group

ranged from 36.545.3 years old. The average of follow-

up time among eight groups ranged from 6.912 years.

The total follow-up person-years are summarized intable 1. The numbers of deceased individuals in the

final three subgroups that were able to be extrapolated

were 118, 391, and 175 for toe amputees, foot or leg

non-amputees, and foot or leg amputees, respectively.

The monthly insured salary for each group was between

NTD $22 96427 328 (equivalent to between USD

$706.60840.90). This salary was comparable to the

average monthly salary of the employed population

of Taiwan (NTD $27 404 or USD $ 843.20) (http://

www.bli.gov.tw). Three groups, consisting of finger

amputees, finger non-amputees, and hand or arm ampu-

tees, appeared to have lower adjusted average monthly

35 436 cases excluded due toincomplete data (N= 4924),multiple disabilities (N=18 605),or injury sites other than upperand lower extremities(N=11 907)

Total number of subjects in thedatabase (N=106 437)

Foot or leg non-amputee

(N=4339)

Subjects included in the analysis(N=71 001)

62 416 cases in which thesurvival couldnt be extrapolated

due to better survival(N= 17 522) or unstable excesshazard (N=44 894)

Toe(s)amputee(N=1016)

Foot or legamputee(N=995)

2235 cases in which the first tenyears cohort did not fit theconstant excess hazard

Figure 1. Flow chart o selection o subjects
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Lin et al

insurance salaries. Up to 70% or more of all disabilities

of the upper extremities were caused by individuals

becoming trapped or caught in machinery, except among

the hand or arm non-amputee subgroup (table 1). In

contrast, more than one-third of injuries in the foot or

leg amputee and non-amputee subgroups were the result

of transportation incidents.

Lie expectancy and loss-o-lie expectancy or three

subgroups

Data analysis revealed a violation of constant excess

hazard for the following five subgroups: (i) toe non-

amputee; (ii) finger amputee; (iii) finger non-amputee;

(iv) hand or arm amputee; and (v) hand or arm non-

amputee. We therefore performed Monte-Carlo extrapo-

lations only on the following three subgroups: toe ampu-

tee; foot or leg amputee; and foot or leg non-amputee.

The estimates of loss-of-life expectancy compared with

the general population among all groups varied between

1.84.9 years, while the health gaps ranged from 5.6

17.7%, as shown in table 2 and figure 2.

Validation o extrapolation

In order to examine the validity of the Monte-Carlo

method in the above three groups, we extrapolated the

cohorts established between 19861995 (10 years) for

an additional 11 years and then compared the results

with actual survival derived from Kaplan-Meyer estima-

tion of the 21 years of follow-up from 19862006 (table

2). The relative bias in the survival estimation for both

the toe amputee and foot or leg amputee groups was

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Table 1b. Frequency distribution o subjects stratifed by demographic actors and non-amputee injury types. [NTD=New Taiwan Dollar]

Fingernon-amputees a

(N=43 690)

Hand or armnon-amputees b

(N=2235)

Toenon-amputees c

(N=1667)

Foot or legnon-amputees d

(N=4339)

Mean SD % Mean SD % Mean SD % Mean SD %

Proportion deceased 6.6 8.2 7.7 9.0

Male gender 76.4 74.0 86.2 78.1

Age at injury (years) 36.5 12.1 45 10.9 39.1 11.9 45.3 10.9

Calendar year o injury

19861990 28.0 16.0 21.8 14.519911995 26.5 7.5 25.1 5.819962000 21.8 13.9 23.8 16.420012006 23.6 62.6 29.3 63.3

Insured salary at injury

50 000 NTD e 0.8 0.0 1.7 0.1

Insured salary (NTD) e 23 301 8996 27 244 9361 26 155 9997 27 328 9582

Injury typeFallen to lower level 0.2 14.1 1.7 15.2

Slipped, tripped, or stumbled 0.3 13.6 0.9 11.4Struck by sliding object orknocked down

1.5 13.9 20.0 18.7

Trapped or caught in machinery 74.4 15.0 47.8 7.7

Cuts, lacerations, or punctures 21.5 4.5 13.9 3.1

Transportation incidents 0.7 32.5 10.1 38.3

Other 1.4 6.3 5.6 5.7

a Total ollow-up person-years=519 911b Total ollow-up person-years=16 092c Total ollow-up person-years=18 004d Total ollow-up person-years=29939e USD 1.00=NTD 32.5 in 2006

Table 2. Frequency distributions and estimates o lie expectancies and loss-o-lie expectancies or three cohorts in which the long-termsurvival could be extrapolated by Monte-Carlo simulation: rom 2150 years. [95% CI=95% confdence interval.]

Foot or leg amputees Toe amputees Foot or leg non-amputees

N Mean % 95% CI N Mean % 95% CI N Mean % 95% CI

Cases 995 1016 4339

Proportion deceased 17.6 11.6 9.0

Mean extrapolation by Monte Carlomethod rom 1021 (years)

17.6 16.818.4 18.6 16.121.1 16.7 11.621.8

Mean survival up to 21 years basedon actual observations calculated byKaplan-Meier method (years)

17.9 17.518.3 18.9 18.519.3 18.1 17.718.5

Relative bias a -1.7 1.6 7.7

Mean lietime survival based onMonte Carlo method (years) b

27.7 24.630.8 32.4 29.335.5 27.0 24.629.4

Mean loss o lie expectancy based onMonte Carlo method (years) b

4.9 2.27.6 1.8 1.34.9 4.5 2.16.9

Health gap 17.7 5.6 16.7

a Relative bias=(dierence between Kaplan-Meier observations and Monte Carlo simulation) / Kaplan-Meier observationsb Based on age- and gender-matched general population

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Figure 2. a, c, and e depict the logit transormation o the survival ratio W(t)between the survival unctions o three groups and those o theage- and gender-matched reerence population generated by the Monte Carlo method. The three groups consist o oot or leg non-amputees, ootor leg amputees, and toe amputees, o which the long-term survival is appropriate to extrapolate by Monte-Carlo simulation; b, d, and depict thepredicted 50-year survival curve or these three groups.

(a) W(t) for foot or leg amputee (b) 50-year survival for foot or leg amputee

(c) W(t) for foot or leg non-amputee (d) 50-year survival for foot or leg

non-amputee

(e) W(t) for toe amputee (f) 50-year survival for toe amputee

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12). To our knowledge, this study is the first to estimate

the long-term survival of people with traumatic dis-

abilities of the upper and lower extremities by using

the semi-parametric extrapolation method. Our results

showed that the life expectancies of workers with toe

amputations, foot or leg amputations, and foot or leg

amputees were 32.4, 27.7, and 27.0 years, respectively,

while their average loss-of-life expectancy were 1.8, 4.9,

and 4.5 years, respectively (table 2). These results were

carefully validated by calculating the relative biases,

which were relatively small except for the estimate

of the foot or leg non-amputee subgroup. However,

because the 95% CI for the loss-of-life expectancy of

the toe amputee subgroup included 0 years, we should

not make any strong inferences based on those results.

The major reason why the people in these three

groups died prematurely was demonstrated in one of our

previous studies (13). This showed that workers with

permanent occupational disabilities of lower extremi-

ties tend to die from diabetes mellitus, cerebrovascular

disease, liver cirrhosis, and chronic renal failure, with

increased standard mortality ratios (SMR) of 7.66,

2.40, 2.07, and 5.09, respectively. In addition, this

study also found that the survival function of workers

with disabilities of the upper extremities could not be

extrapolated using this method because of the violation

of the assumption of constant excess hazard. In fact,

these workers still suffered from increased mortalities

from suicide, transportation fatalities, and other injuries,

but their causes of deaths did not constitute a constant

mortality rate for their survival curve.This study has at least two major implications. First,

it quantified the amount of loss-of-life expectancy due

to occupational injury, which can be used in future cost-

effectiveness analyses aimed at helping prevent such

injuries. Although this study did not show the lifetime

medical cost for such injuries, we have demonstrated

that the prevention of a case of foot or leg amputation

can save about 4.9 life-years, while preventing a foot

or leg injury that does not result in amputation can save

about 4.5 life-years. Second, while most countries current

system of workers compensation provides some form of

income supplementation for injured workers, none haveincorporated compensation for the reduction in workers

life expectancy. Based on the theory of justice to preserve

ones own capabilities and possibly to compensate others

for the loss-of-life expectancy (14, 15), we recommend

that future improvements to compensations systems take

reduction in life expectancy into account.

Study limitations

Although we have used the most comprehensive national

data currently available in Taiwan, this study has some

limitations. First, life expectancy is also a function of

comorbidity. Besides age, gender, and the occurrence of

occupational disability of the upper or lower extremities,

there are many other risk factors, including underlying

chronic diseases, lifestyle, and socioeconomic status,

that determine peoples life expectancy. As subjects

included in this study were young or middle-aged work-

ers with an average age ranging between 36.545.3

years (table 1), the extent to which comorbidity of

chronic diseases confounds our results might not be

very large.

Second, all subjects were actively employed; they

were healthy workers who would have had better base-

line health conditions and reduced mortality compared to

the general population had the work-related injuries not

occurred (16, 17). Thus, our study mildly underestimated

the reduction in life expectancy associated with disability.

Moreover, the lifetime extrapolation is based on the life

tables established by current and previous vital statistics.

Because health technology continues to improve, such

a method could easily underestimate the actual survival

of workers in the future, including those with permanent

occupational disabilities of the lower extremities. Thus,

our estimations of life expectancy and loss-of-life expec-

tancy for these workers are conservative and must be

constantly adjusted to reflect the most current data.

Concluding remarks

Workers who sustained occupational injuries and pos-

sessed permanent disabilities of the foot or leg were

found to have a loss-of-life expectancy of 4.9 yearswhen their injuries resulted in amputation and 4.5 years

when they did not. We have also demonstrated that the

semi-parametric extrapolation method is feasible and

accurate for predicting life expectancy and expected

years-of-life-lost for cohorts with permanent occupa-

tional disabilities. In the interest of fairness, we rec-

ommend that such losses be taken into consideration

in workers compensation systems. In addition, these

results can be used to assess the cost-effectiveness of

preventive occupational health services.

Acknowledgements

This study is supported partially by a grant from the

Institute of Occupational Safety and Health (IOSH)

of the Council of Labor Affairs, Executive Yuan of

Taiwan (No. IOSH97-M101) and another grant from

the National Science Council (NSC 96-2628-B-002-

071-MY3).

We are indebted to the Bureau of Labor Insurance

of Taiwan for kindly providing the database of workers

compensation during 19862006.

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Received for publication: 24 September 2010
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